1. Field of the Invention
The present disclosure relates to a back light unit providing direction controllable collimated light beam and a three-dimensional display using the same. Especially, the present disclosure relates to a back light unit providing highly collimated light beam and controlling the direction of the collimated light beam and an auto-stereoscopy type three-dimensional display using the same.
2. Discussion of the Related Art
Recently, many technologies and researches for making and reproducing the 3D (three dimensional) image/video are being actively developed. As media relating to the 3D image/video is a new concept for virtual reality, it can improve the visual information, and it will lead the next generation display devices. The conventional 2D (two dimensional) image system merely suggests the image and video data projected to plan view, but the 3D image system can provide the full real image data to the viewer. So, the 3D image/video technologies are the more efficient image/video technologies.
Typically there are five methods for reproducing 3D image/video: the stereoscopy method, the auto-stereoscopy method, the volumetric method, the holography method, and the integral imaging method. Among them, the holography method uses a laser beam so that it is possible to observe the 3D image/video with naked eyes (i.e., without the need for glasses). The holography method is the most ideal method because it has an excellent visual stereoscopic property without any fatigue of observer.
To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). If these two beams are coherent, optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram. The central goal of holography is that when the recorded grating is later illuminated by a substitute reference beam, the original object beam is reconstructed (or reproduced), producing a 3D image/video.
There was a new development of the computer generated holography (or CGH) that is the method of digitally generating holographic interference patterns. A holographic image can be generated e.g. by digitally computing a holographic interference pattern and printing it onto a mask or film for subsequent illumination by suitable coherent light source. The holographic image can be brought to life by a holographic 3D display, bypassing the need of having to fabricate a “hardcopy” of the holographic interference pattern each time.
Computer generated holograms have the advantage that the objects which one wants to show do not have to possess any physical reality at all. If holographic data of existing objects is generated optically, but digitally recorded and processed, and brought to display subsequently, this is termed CGH as well. For example, a holographic interference pattern is generated by a computer system and it is sent to a spatial light modulator such as LCSLM (Liquid Crystal Spatial Light Modulator), then the 3D image/video corresponding to the holographic interference pattern is reconstructed/reproduced by radiating a reference beam to the spatial light modulator. FIG. 1 is the structural drawing illustrating the digital holography image/video display device using the computer generated holography according to the related art.
Referring to FIG. 1, the computer 10 generates a holographic interference pattern of an image/video data to be displayed. The generated holographic interference pattern is sent to a SLM 20. The SLM 20, as a transmittive liquid crystal display device, can represent the holographic interference pattern. At one side of the SLM 20, a laser source 32 for generating a reference beam is located. In order to radiate the reference beam 90 from the laser source 32 onto the whole surface of the SLM 20, an expander 40, and a lens system 50 can be disposed, sequentially. The reference beam 90 generated from the laser source 32 is radiated to one side of the SLM 20 passing through the expander 40 and the lens system 50. As the SLM 20 is a transmittive liquid crystal display device, a 3D image/video corresponding to the holography interference pattern will be reconstructed/reproduced at the other side of the SLM 20.
The holography type 3D display system according to the FIG. 1 comprises a back light unit BLU for generating a reference light (i.e., a light beam) 90 satisfying certain condition and for providing the back light from the reference light 90 to a SLM (Spatial Light Modulator) 20 having a large diagonal area. In FIG. 1, the back light unit BLU comprises light source 30 for generating the reference light 90, an expander 40 and a lens system 50 which have relatively large volume. In a case that the holography 3D display system having this back light unit BLU is configured, the brightness (or luminescence) of the back light is not evenly distributed over the large area of the SLM 20 because the light brightness distribution of the light source 32 has the Gaussian profile. Furthermore, in order to reduce the high-level mode noise of the diffracted light of the reference light 90 which may cause the image noise, the incident light may have certain incident angle to the SLM 20. In that case, the collimation property may be damaged.
For overcoming this problem of the related art, a back light system for maintaining the collimation property of the incident light to the SLM even if it is entering into the SLM with an incident angle enough for reducing the 0th mode noise of the diffracted light is under developed. There is a system, for example, in which a collimation lens is used. FIG. 2A is a schematic view illustrating a back light unit generating the collimated light beam using a collimation lens.
Referring to FIG. 2A, a point light source is placed at the light source 30, and a collimation lens CL is placed at the focal point of the collimation lens CL from the light source 30. Then, the light radiated from the light source 30 can be a collimated light beam 100 after passing the collimation lens CL. That is, the radiated light in the collimated light beam 100 is substantially parallel to one another after passing the collimation lens CL. This collimated light can be used for a reference light in the holography 3D display.
In the most cases of the holography 3D display systems, the reference light should enter into the spatial light modulator with certain incident angle. The light diffraction elements, such as SLM, can make the 0th mode image, 1st mode image and the higher-level mode images. The 1st mode image and higher level mode images are considered noise. In order to eliminate or reduce the higher-level mode images, it is preferable to radiate the light into the light diffraction element with certain incident angle.
To do so, in the back light unit shown in the FIG. 2A, the position of the light source 30 can be shifted to one side from the center of the light axis 130 to make the incident angle. FIG. 2B is a schematic view illustrating the back light unit generating the collimated light propagating with certain incident angle by the collimation lens CL.
Referring to FIG. 2B, the point light source 30 is moved upward from the light axis 130 so that the incident angle from the moved point to the center of the collimation lens CL can be α. Then, theoretically, as the dotted line of the FIG. 2B, the collimated light 100 can propagate to the inclined direction with α° from the parallel direction to the light axis 130 which represents the ideal path for the collimated light 100. However, the actual path of the inclined light cannot be collimated (or parallel) light having incident angle α, as repsented by the solid line of FIG. 2B. As a result, the collimated light from the back light unit cannot be illustrated to a target area with evenly distributed brightness but rather has a distorted area profile.